Conventionally, in a cold rolling operation of a steel sheet, regardless of a cold rolling apparatus, such as a completely continuous cold tandem mill, a continuous tandem mill arranged subsequently to a pickling line, or a single-stand reverse mill, the steel sheet heated to a level of room temperature that is at most 40° C. is cold-rolled. This is because, even after considering that the deformation resistance of the steel sheet lowers along with the increase of a steel-sheet temperature, a demerit becomes large compared with a merit obtained by increasing the temperature of the steel sheet that is a material to be rolled. For example, as a merit obtained by increasing the temperature of the steel sheet, the decrease of the rolling power along with the decrease of the deformation resistance of the steel sheet can be designated. However, in the cold rolling operation of the steel sheet, this merit can be almost disregarded. On the other hand, there exists a large demerit attributed to the temperature increases of the steel sheet, such as the extremely large cost loss for increasing a steel-sheet temperature, or the handling problem of a hot steel sheet with respect to a labor environment.
When the steel sheet heated to a level of room temperature is cold-rolled as mentioned above, there exists the possibility that edge cracks occur in an end portion (hereinafter, referred to as “edge portion”) in the width direction of the steel sheet in the process of cold rolling. Particularly, a material difficult to be rolled, such as a silicon steel sheet containing 1% or more of silicon, a stainless steel sheet, or a high carbon steel sheet, is a brittle material as compared with a general steel sheet and hence, when the material difficult to be rolled is heated to a level of room temperature and cold-rolled, the edge cracks remarkably occur. When the extent of the edge crack is large, there exists the possibility that the steel sheet is broken from the edge crack as a starting point in the process of cold rolling.
As a method of overcoming such problems, for example, Patent Literature 1 discloses a method for cold-rolling a silicon steel sheet in which the silicon steel sheet at its edge portion heated to 60° C. or higher (ductile brittle transition temperature) is, in cold-rolling the silicon steel sheet, supplied to a rolling mill as a material to be rolled. Furthermore, Patent Literature 2 discloses a pair of induction heating devices each using a C-shaped inductor (heating inductor) as a means for increasing the temperature of an edge portion of a steel sheet by induction heating. The induction heating device described in Patent Literature 2 is constituted such that each of both the edge portions of the steel sheet in the width direction (hereinafter, referred properly to as “sheet width direction”) are inserted into a slit of the C-shaped inductor in a vertically sandwiched and spaced apart manner, a high frequency current is sent to the coil of the C-shaped inductor from a power unit to apply magnetic fluxes to the edge portions in the thickness direction of the steel sheet (hereinafter, referred properly to “sheet thickness direction”) and generate an induced current in the edge portions, and the edge portions are heated with the Joule heat that occurs by the induced current.
Here, in order to heat the edge portion of the steel sheet to a predetermined temperature, it is necessary that the length of the edge portion of the steel sheet overlapping with the C-shaped inductor whose slit inserts the edge portion thereinto in a vertically sandwiched and spaced apart manner in the sheet thickness direction (hereinafter, referred to as “overlapping length”) assume a predetermined value by setting the position of a carriage that supports the C-shaped inductor depending on the sheet width of the steel sheet. However, in an actual operation, a steel sheet moves in a meandering manner in the sheet width direction by a poor centering accuracy or a poor flatness of the steel sheet thus changing the overlapping length. When the overlapping length decreases, the occurrence of an eddy current that obstructs the flow of the magnetic flux decreases and hence, even when a power factor deteriorates to increase a wattless current and a high frequency current that flows into the coil of the C-shaped inductor increases to a rated value, it is impossible to achieve a predetermined output. As a result, there exists the possibility that the underheat of the edge portion occurs. There also exists the possibility that the situation of excessively heating a part of the edge portion (abnormal local heating) arises. In the case of the underheat, edge cracks occur in the edge portion while cold-rolling the steel sheet. The edge cracks cause the fracture of the steel sheet in the process of cold rolling as described above. On the other hand, in the case of the abnormal local heating, edge waves attributed to a deformation by a thermal stress occur in the edge portion of the steel sheet. When the extent of the edge wave is large, there exists the possibility that a drawing fracture occurs in the steel sheet in the process of cold rolling and hence, it is difficult to cold-roll the steel sheet stably. In this manner, when the edge portion of the steel sheet to be cold-rolled is heated to a predetermined temperature by induction heating, it is extremely important to control the overlapping length to an optimal value.
Here, as a conventional technique with respect to the control of the overlapping length mentioned above, for example, there is disclosed an induction heating device provided with a heating coil that heats edge portion of a steel sheet transferred, a coil carriage body on which the heating coil is mounted, a movement mechanism that moves the coil carriage body in the direction orthogonal to the movement direction of the steel sheet, and guide rollers that are attached to the coil carriage body and brought into contact with the edge portion of the steel sheet (refer to Patent Literature 3). The induction heating device described in Patent Literature 3 operates the movement mechanism so that the guide rollers are brought into contact with the edge portion of the steel sheet while induction-heating the steel sheet, and always keeps the relative position relation between the steel sheet and the heating coil constant. On the other hand, there is disclosed a method of induction-heating control in which carriages each of which moves in the direction orthogonal to the movement direction of the steel sheet are located at the respective left-and-right side positions of the line through which the left-and-right edge portions of the steel sheet pass, inductors each of which inserts the edge portion of the steel sheet thereinto in a vertically sandwiched manner are arranged on the respective carriages located at left-and-right positions, and an automatic position controller of the carriage controls the overlapping length between the edge portion of the steel sheet and the inductor to heat the edge portion of the steel sheet (refer to Patent Literature 4). In the method of induction-heating control described in Patent Literature 4, the high frequency current that flows into the heating coil of each of the inductors located at left-and-right positions is detected, the deviation of an electric current value that is generated by the change of the overlapping length due to the meandering movement of the steel sheet is obtained, and a carriage position correction value is obtained based on a relation between a deviation electric current value stored in advance and a carriage position correction amount of the inductor that is required to set the deviation electric current value to zero. Subsequently, the carriage position correction value is subtracted from a carriage position initialized value on the large electric current value side of the carriage and, at the same time, the carriage position correction value is added to a carriage position initialized value on the small electric current value side of the carriage to obtain a carriage correction position on either side. Thereafter, the carriage correction position on the either side that is calculated as mentioned above is output to the automatic position controller of each carriage on either side and hence, the position of each carriage on the either side is corrected by the automatic position controller. Due to such a constitution, the overlapping length between each of the left-and-right edge portions of the steel sheet and each inductor on either side is controlled.